1. Field of the Invention
The present invention relates to a method for controlling a magnetic resonance system for implementing a magnetic resonance measurement in at least one defined volume area of interest inside an examination object, the magnetic resonance system being of the type having a radio-frequency antenna having a number of resonator elements that can be excited in different transmit (transmission) modes to generate linearly independent radio-frequency field distributions an examination volume that includes the examination object. In addition, the invention relates to a magnetic resonance system, suitable for implementing such a method, having a corresponding radio-frequency antenna and to a computer program product which, in order to implement the method, can be loaded into a memory of a programmable control unit of such a magnetic resonance system.
2. Description of the Prior Art
Magnetic resonance tomography is a now widely used technology for obtaining images of the interior of the body of a living examination object. In order to obtain an image by this method, the body or body part to be examined of the patient must be exposed to as homogeneous as possible a static main magnetic field (usually called B0 field) which is generated by a main field magnet of the magnetic resonance apparatus. During the acquisition of magnetic resonance images, rapidly switched gradient fields, which are generated by gradient coils, are superimposed on this main magnetic field for location coding. In addition, radio-frequency pulses of a defined field strength are irradiated into the examination object by radio-frequency antennae. The magnetic flux density of these radio-frequency pulses is usually designated B1. The pulse-shaped radio-frequency field is therefore generally also abbreviated as the B1 field. By means of these radio-frequency pulses, the nuclear spins of the atoms in the examination object are excited such that they are deflected from their position of equilibrium parallel to the main magnetic field B0 by a so-called “excitation flip angle” (in general, also abbreviated to “flip angle”). The nuclear spins then precess around the direction of the main magnetic field. The magnetic resonance signals generated thereby are acquired (detected) by radio-frequency receiving antennas. The receiving antennas can be either the same antennas that are also used to emit the radio-frequency pulses, or separate receiving antennas. The magnetic resonance images of the examination object are finally created on the basis of the acquired magnetic resonance signals. Each pixel in the magnetic resonance image is assigned to a small body volume, called a “voxel”, and each brightness or intensity value of the pixels is linked to the amplitude of the magnetic resonance signal received from this voxel. The correlation between a resonantly irradiated radio-frequency pulse of field strength B1 and the flip angle □ achieved therewith is given by the equation
                    α        =                              ∫                          t              =              0                        τ                    ⁢                      γ            ·                                          B                1                            ⁡                              (                t                )                                      ·                          ⅆ              t                                                          (        1        )            wherein γ is the gyromagnetic ratio, which for most nuclear spin examinations can be considered a fixed material constant, and τ is the effective duration of the radio-frequency pulse. The flip angle achieved by an emitted radio-frequency pulse and thus the strength of the magnetic resonance signal consequently depends not only on the duration of the pulse but also on the strength of the irradiated B1 field. Spatial fluctuations in the field strength of the exciting B1 field therefore lead to undesired variations in the magnetic resonance signal received, and these variations can distort the measurement result.
Disadvantageously, however, it is precisely where magnetic field strengths are high—which is necessarily the case because of the main field B0 that is needed in a nuclear spin tomograph—that radio-frequency pulses display non-homogeneous penetration behavior in conductive and dielectric media such as, e.g., body tissue. The result is that the B1 field can vary greatly within the measurement volume. Particularly in the ultra-high field range with magnetic field strengths B0≧3 T, significant influences of radio-frequency penetration behavior on image quality are observed. Due to B1 focusing and shielding effects, the flip angle of the high-frequency pulses becomes a function of the location. Contrast and brightness of the recorded magnetic resonance images thus vary in the imaged tissue and can in the worst cases result in pathological structures not being visible.
As a promising approach toward solving this problem, multi-channel transmit coils, also called transmit arrays, are currently under discussion. These are radio-frequency antennas of the type described in the introduction formed by a number of resonator elements or antenna elements that can be activated individually or in groups, i.e. in different transmit configurations. This is possible, for example, if the individual resonator elements are electromagnetically decoupled from one another and can be activated with an individual amplitude and phase over separate radio-frequency channels. Different radio-frequency distributions form in the examination volume of the antenna depending on the amplitudes and phases with which the different transmit configurations are to be excited. For example, it is possible to generate with an antenna having N electromagnetically decoupled and individually controllable resonator elements, N linearly independent field distributions. A simple example of this is a birdcage resonator having rods that can each be activated individually with regard to their amplitude and phase. Each of these rods generates, independently of one another, a B1 field, the B1 fields of the individual rods being superimposed in relation to the overall field distribution.
Instead of looking at the separate resonator elements individually, different “collective excitation modes” can be excited individually using an antenna of this type. In order to activate such collective modes, also called “transmit modes” or “field modes”, for example, a fixed-output mode matrix (e.g. a Butler matrix) can be installed in the hardware used for activating the antenna elements. Alternatively, appropriate activation of the individual antenna elements can be achieved for by software.
Through individual settings of the amplitude and phase of the radio-frequency pulse emitted by each transmit configuration, influence can be exerted on the spatial distribution of the B1 field with the aim of generating a radio-frequency field in the object or in the examination volume that is as homogeneous as possible. Magnetic resonance apparatuses of this type are described for example in U.S. Pat. No. 6,043,658 and DE 10 2004 045 691 A1.
From DE 10 2004 013 422 A1, a method and a magnetic resonance system for homogenizing a B1 field are known. Homogenization of the B1 field is achieved in iteration steps. In a first iteration step, measurement data is acquired that represents a B1 field distribution in at least one part of an examination volume, a B1 homogeneity analysis subsequently being implemented automatically on the basis of the acquired measurement data. Automatic selection of a defined homogenization action from a number of possible homogenization actions is then implemented on the basis of the B1 homogeneity analysis. A selected homogenization action is subsequently implemented in order ultimately to homogenize the B1 field.
A hitherto unresolved problem, however, is that of determining the transmit parameters for the individual antenna elements so that is as homogenous as possible that a B1 distribution is actually achieved in the patient, or at least in the region of interest (ROI) for the desired imaging. One possible approach to determining the parameters could take the form of a distribution of the B1 field with regard to the magnitude and phase thereof being transmitted for each individual resonator element. An overview display would then have to be determined with all the resonator elements being active. An optimization region (e.g. the ROI) must then be identified and, furthermore, the activation parameters computed for the homogenized excitation. Such measurements are, however, extraordinarily time-consuming. The total adjustment time can take up to 10 minutes. This method is consequently not very suitable as an adjustment method in practice.